November 2008 Archives

Astronomy 101 - Lesson 9 - The Sky In Motion (2)

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In the last instalment, I described the effects of the rotation of the Earth on how we see the stars above us (and on which stars we see and which ones we don't). That's all very well, but there's one other major type of movement of our planet that affects, and much, the way the sky looks like: we also orbit our Sun, and complete one orbit in one year.

The orbit of the Earth around the Sun has one major, immediate effect: the position of the Sun on the sky (against the background of stars) changes as the year goes by; the Sun will complete a full circle of the sky in a year, which means that its position changes by about one degree a day (360 degrees in a circle, 365 days in a year) — one degree is about twice the apparent diameter of a full moon. Of course, this is not directly visible: we can't see the stars around the Sun during the day. What we will notice is that the field of stars that is visible at night will subtly change from day to day — the whole field will seem to move westwards by about one degree a day.

One important thing to notice is that this apparent movement of the Sun relative to the stars (or of the stars relative to the Sun) is not parallel to the Equator: it is, in fact, tilted by approximately 23.5° (the exact angle changes over time; nowadays it's more like 23.44°, or 23°26'). This tilt has one very significant effect on Earth, but that's the subject of our next article: the four seasons.

Ecliptic, by Tau Ľolunga; click to enlargeThe apparent path of the Sun on the sky has a special name: it's called the ecliptic. This is also the path followed (with varying degrees of precision) by the other planets in their path on the sky, as we'll see in the future. The ecliptic goes through several constellations — to be more precise, twelve. These are called the zodiacal constellations, or the constellations of the zodiac (from the Greek meaning "circle of animals" — most zodiacal constellations are represented by animals). One can see how twelve constellations resulted not only in the twelve astrological signs, but also in the twelve months of the year.

There are four special points on the ecliptic that are worth mentioning:

  • the solstices: these are the points of maximum north or south drift (or declination) of the Sun; the northern solstice happens in June, while the southern solstice happens in December
  • the equinoxes: these are the points of minimum declination, that is, where the Sun crosses the celestial Equator; they are called the autumnal equinox (which happens in September), when the Sun crosses the Equator moving northward, and the vernal equinox (in March), moving southward.

And that's it for today. As I mentioned, in the next article we'll explore in more details the effects of the tilt in our orbit and the significance of the solstices and equinoxes.

More exoplanet images?

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2008-11-22-bpictoris.jpgNow, this is not 100% confirmed, but it does look like we've got another image of an extrasolar planet: this one (if real) is orbiting the star β Pictoris, a very young star 70 light-years away from us.

The potential discovery comes from a new analysis of images taken in 2003 with ESO's Very Large Telescope. The images were processed to subtract from them the light coming directly from the star, allowing scientists to see objects that are around it; this showed a very distinct point of light very close to the star and in the same plane as the dust ring that surrounds it, but we can't still rule out the possibility that this is a background or foreground object instead of something actually in the neighbourhood of the star.

If this is a real planet, it is closer to its star than the other ones imaged previously, being approximately as far from it as Saturn is from the Sun; it would be a very large planet, though, about eight times as massive as Jupiter.

New observations might prove this object to be a planet (by showing its movement around the star, presumably), so we will definitely hear more about β Pictoris in the future. More details at the ESO press release.

Carnival #80 is up

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The Thanksgiving edition of the Carnival of Space is up at Starts With a Bang; this is 80th edition of the carnival already!

As at any good Thanksgiving dinner, this one has touching stories, heated discussions (with several bloggers expressing differing opinions on the past existence of oceans on Mars) and even tips about the upcoming winter (northern hemisphere winter, of course — although, with the snow now falling in the mountains a few hundred km from here, it's feeling like winter in Melbourne as well...). Don't miss it!

Extrasolar planets imaged directly

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2008-11-14-fomalhaut.jpgThis has been the talk of the Internet today, so I might well write about it as well... for the first time, scientists were able to capture images (in visible light, no less) of not one, but four extrasolar planets orbiting around two separate "normal" stars (we had already seen images of a planet orbiting a brown dwarf star).

First, we have star HR8799, a young star that is a bit larger than our Sun (1.5 times as massive and 5 times as bright) and that lies 130 light years away in the constellation Pegasus. Images taken with the Keck and Gemini North telescopes in Hawaii show three large planets orbiting this star; one is about seven times and other two are 10 times as massive as Jupiter. They orbit the start at distances ranging from 24 to 67 AU (the planetary limits of our own solar system are around 30 AU).

Apart from the historical value of directly imaging these planets, this event is significant for other reasons: this star is very similar to our own, and these large planets are orbiting it at a large distance, leaving space closer to the star for small rocky worlds; in other words, this might be a solar system similar to our own, which is something of a rarity among the hundreds of other solar systems we've already found.

Secondly, we have Fomalhaut, a larger star (2.3 times as massive and 16 times as bright as the Sun) 25 light years away in the constellation Pisces Austrinus, the southern fish. It's been known for a while that this star is surrounded by a large dust disk; in fact, the very sharp inner edge of this disk was a clue that there was a planet there, cleaning out debris just inside the disk. And, indeed, Hubble images do show a bright planet located there, in a very wide orbit around the star. The planet seems to be about twice as massive as Jupiter, and its brightness may indicate it is surrounded by a very large ring system.

The planet orbits the star once every 872 years at a distance that is almost four times the distance from Neptune to our Sun, but since Fomalhaut is brighter than our Sun its appearance would be similar to how the Sun appears when seen from Neptune. Just as is the case with HR 7899, Fomalhaut is a very young star and its solar system is still being formed, which helps in the detection of the planets: they're still hot enough that they radiate brightly in the infrared.

Both stars are visible with the naked eye from a dark sky site, and Fomalhaut should be easily visible even from a urban setting. If you are in Melbourne (or any place at the same latitude), Fomalhaut will be almost directly overhead today soon after sunset — it's bright enough that you can't really miss it. HR 7899 will be a bit harder to find; Pegasus will be visible on the northern sky in the middle of the night, and the Lowell Observatory press release has a diagram that will help you find the right star (it's likely you will need a binocular or small telescope, though).

For more details and images, in addition to the links in the text above, see the (very enthusiastic) Bad Astronomy blog and Centauri Dreams.

Phoenix mission ends

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The Phoenix Mars Lander has stopped transmitting on 2 November, and NASA has declared the mission to be over. This was expected — Phoenix was never supposed to survive the Martian winter that is starting now, and will likely be fully encased in solid ice (probably CO2 ice) during the long winter months. It failed due to the diminishing day light and the increasingly cold temperatures.

Dave Moshen Mosher, at the Discovery Channel's Space Disco blog, has a good post about the mission with links to many pictures and videos from the mission. It's a shame that the microphone installed on the probe ended up never working...

There's a slim chance that the probe might come back to life in the next summer, after the polar ice cap melts back and it gets enough sun light. It is very unlikely that this will happen, though — even though the probe is named "Phoenix"...

365 Days of Astronomy

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As I've already mentioned before, 2009 will be the International Year of Astronomy, and many activities are being scheduled for the whole year — both online and offline. One of this is the 365 Days of Astronomy podcast, a project that will publish one short podcast every day during the whole year.

A "test podcast" was already published, but the feed will actually go live on the 1st of January (for some time zone; it may be on the 31st of December or the 2nd of January depending on where you are...), and the project will depend heavily on contributions from interested listeners/readers — that means you. It doesn't matter that you don't have a podcast, or that you've never produced audio material: you can still participate. Go to the website, read the information that is there and volunteer to help in any way you can. Even if it is by just mentioning the project in your own blog.

2009 will be an interesting year!

Close to the Moon

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Moon close to JupiterIf you like seeing close appearances of bright objects in the sky, this week is being very good for you (assuming the weather in your location is being better than here). Last Saturday, the (very thin crescent) Moon was very close to Venus soon after sunset; I went out to try to look at it, but the only clouded area on the sky was the western horizon up to some 45 degrees... so, no luck there.

Tonight it is Jupiter's turn, with the Moon (now considerably less thin) very close to it and both setting around midnight. Jupiter is not as bright as Venus but it's no less spectacular (especially considering that the sky will be darker as it sets later). The sky is fully clouded here, I hope people have better luck in other locations...

(the times above are for Melbourne, and things will look different — that is, the Moon closer or farther away from Jupiter — depending on where you are)

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This page is an archive of entries from November 2008 listed from newest to oldest.

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